Devices are needed which can provide both quantitative information about the topography of a sample through atomic force microscopy and optical image information.
Atomic force microscopes have been designed which attach directly to the lens turret of a standard optical microscope. For example, the DUALSCOPE.TM., manufactured by DME (Danish Micro Engineering) is an AFM with a built-in objective lens. The DUALSCOPE.TM. mounts directly onto a lens turret of an optical microscope.
The DUALSCOPE.TM. uses an optical beam bounce detection system. A significant disadvantage of coupling an AFM which uses an optical beam bounce detection system to an optical microscope, such as the DUALSCOPE.TM., is that the laser beam obscures and distorts the optical view. Laser light from the detection system scatters light both from the cantilever and the sample which distorts the field of view.
A further disadvantage of using an optical beam bounce detection system is the use of the lenses for optical imaging to also focus laser light onto the back of the cantilever. Laser light in an AFM is typically red light of a specific wavelength, whereas an optical microscope is designed to focus white light. Two different focal lengths are therefore required, one for the red light which is focused onto the back of the cantilever, and another for white light which must be focused onto the sample which is about 2 to 4 .mu.m below the cantilever due to its thickness and the length of the tip. If the optics used in the AFM are designed primarily to focus the laser light, the optical clarity of the optical view is degraded. If the optics are designed primarily to provide a high quality optical view, the degree that the laser light is focused on the cantilever is compromised. As a result, the spot on the cantilever is larger and more diffuse. The unfocused laser spot leads to greater laser light scattering from the cantilever which degrades the optical view. In addition, there is greater laser light scattering from the sample, which is not distinguishable from light reflected from the cantilever. As a result, the photodetector detects both sources of scattered light, leading to interference and other noise in the SPM image.
In the case of the DUALSCOPE.TM., the optical view is focused at infinity, also referred to as "infinity corrected." This allows the device to be installed on different standard optical microscopes. However, by focusing the optical view at an image at infinity, both the red laser light and white light for the optical view are focused at the same point. This inherently requires that the optical view and/or the laser focus be degraded.
The DUALSCOPE.TM. provides simultaneous SPM and optical viewing by incorporating optics within a piezoelectric tube scanner. Because the optics are positioned in the piezoelectric tube scanner, the optics are scanned with the SPM. This design also severely compromises the optical quality of the optical view that is provided. For example, the DUALSCOPE.TM. does not allow simultaneous use of all optical modes of an optical microscope, including other optical contrast techniques such as confocal microscopy, phase contrast microscopy, differential interference contrast microscopy, and Hoffman/modulation contrast microscopy.
Positioning the optics within the scanner tube, as in the DUALSCOPE.TM., also limits the types and quality of lenses which may be used. A scanner tube is too long to allow the AFM to be mounted below a standard working distance objective lens. The longest standardly available working distance objectives which mount on standard optical microscope turrets typically have working distances on the order of 1" and a 10-20.times. magnification (limited by the Rayleigh criterion, i.e., diffraction limited) and are very expensive. Therefore on-axis optics must be placed within or below the scanner tube and must also be scanned. These constraints limit the diameter of the lenses which then limit the numerical aperture of the lens. By limiting the numerical aperture of the lens, the resolving power of the optics and the depth of focus are limited which leads to more spherical aberrations in the image. Having to position the optics within the tube scanner also limits how heavy the lens can be which affects the resonant frequency of the AFM, which is preferably high. The quality of the lenses which may be used is also limited by the constraint of having to position the optics within the tube scanner. For example, for a given field of view, the resolution, depth of focus, and severity of spherical aberration all are worse with smaller diameter lenses. In addition, needing to position the optics within the tube scanner as opposed to using optics which are already being used on the optical microscope increase the costs associated with the device.
Another disadvantage associated with positioning the optics within the scanner tube is the degree of magnification which the optics provide. For example, the degree of total magnification provided by the series of optics used in combination with the DUALSCOPE.TM. is specified as 25.times.. A higher level of magnification is needed for many applications. For example, semiconductor defect inspection systems and defect review systems require a lateral resolution of 1 .mu.m or less which requires 400-1000.times. magnification. Current semiconductor defects, and even the semiconductor devices themselves, can be 1 .mu.m or less in size. The optical resolution provided by an AFM such as the DUALSCOPE.TM. which uses optical beam bounce can be on the order of greater than 5 .mu.m. With an optical resolution on the order of 5-10 .mu.m, these devices poorly resolve the cantilever itself, let alone the sample.
A further disadvantage of positioning the optics within the scanner tube is that the optical view tracks the motion of the probe. As a result, the probe remains fixed in the field of view. Hence, it is not possible to scan a probe within a stationary optical field of view. This limitation makes probe positioning difficult.
Another AFM which has been developed for use in combination with an optical microscope is the ULTRA OBJECTIVE.TM., manufactured by Surface Imaging Systems. The ULTRA OBJECTIVE.TM. does not include a built-in objective lens which mounts directly onto a lens turret. As a result, the ULTRA OBJECTIVE.TM. does not enable simultaneous SPM and optical imaging. Instead, it is necessary to switch between the AFM and an objective lens.
The ULTRA OBJECTIVE.TM. uses laser interferometry to detect cantilever deflection and is designed to be compatible with only Zeiss optical microscopes.
Given the large number of instruments in existence with optical microscopy capabilities, a need also exists for a device which can introduce atomic force microscopy into these existing instruments in order to avoid the capital costs associated with purchasing a new instrument having combined atomic force microscopy and optical microscopy capabilities. The device should rapid switching between atomic force microscopy and optical imaging and should preferably enable simultaneous atomic force microscopy and optical imaging. The device should also preserve the optical clarity, functionality, and resolution that is provided by the instrument.
These and other advantages are provided by the AFMs of the present invention.